Brake bleeding question

[rwolcott23]

We have bled our brakes though the pedal is still extremely spongy (pretty sure there is no air in there). After inspecting everything it is apparent that the pistons are being sucked back into the calipers when the pedal is released. What is the best cure for this? We are currently using two Tilton MCs with a mechanical bias bar.

Thanks,
Bob

[rwolcott23]

We have bled our brakes though the pedal is still extremely spongy (pretty sure there is no air in there). After inspecting everything it is apparent that the pistons are being sucked back into the calipers when the pedal is released. What is the best cure for this? We are currently using two Tilton MCs with a mechanical bias bar.

Thanks,
Bob

[Cement Legs]

If the pedal is still spongy I'm tempted to say that there is still air in the system somewhere. Have you mounted the calipers in their intended orientation. If not they might not bleed properly if there are cavities where air can accumulate under your custom setup.

If you are not running steel braided lines you will loose some pressure due to expansion. What is the performance like? Are you getting good braking g's and just noticing the sponginess and a side effect? There are some systems that will always feel spongy compared to others. Did you guys go back and re-tighten all lines and connections? (use flare nut wrench!)

If all else fails and you want to know for sure if there is a part failure somewhere you can keep the lines connected but remove the components and bench bleed the system. This way you can twist the parts around in the air to make sure that all of the air is being purged.

Hope the ideas help http://fsae.com/groupee_common/emoticons/icon_wink.gif

[rwolcott23]

The system was filled and bled for the first time last night and there very well may still be air bubbles in the line. I was told this morning that the pistons appear to retract into the caliper when the brake pedal force is removed. This increased travel would create a spongy pedal feel as well. I was told a possible remedy would be a residual pressure valve.

Have you heard of anything like this creating poor pedal feel when air bubbles are not the culprit?

Our 2004 braking setup was very solid even with the calipers flexing .06" at 700 psi. The new calipers will have 25% of the flex (at 850 psi), smaller total piston area (2in^2 vs. 2.4in^2 per half), and four pistons instead of two, so the fluid demands should be a fraction of our 2004 system's. The master cylinder sizes are not significantly different than before either.

Thanks,
Bob

[Denny Trimble]

If air isn't the problem, brake pedal mount stiffness could be.

I suspect air in the MC or calipers.

[rwolcott23]

Okay, here is what I have found out. The master cylinders are mounted below the calipers so when the pressure is removed, the weight of the fluid causes a negative pressure in the calipers and retracts the pistons. This creates too much piston to pad clearance and the result is a spongy pedal.

The residual pressure valve maintains a pressure of 2 psi and prevents this from happening. I guess this is very common in racing applications.

Bob

[Nick McNaughton]

We've had fun with something similar, there might be a few things happening...

When the caliper pistons retract, it means that it's easier to pull the pistons in than it is to pull fluid from the reservoir. If it's in the master, there's something wrong with the fluid path between the seals and the reservoir port. Likely causes are - incorrectly shimmed master cyls, the master seals are too stiff to collapse or are the wrong type, or the piston in the master doesn't have enough provision for fluid flow around or through it. Check if the pad retraction happens at both front and rear calipers, or if just one circuit is a problem. If you're running custom calipers, make sure the piston seal you're using doesn't have a huge tendancy to pull the pistons back to fully retracted by being really stiff. If this is the problem, try preloading it lightly with a spring behind the piston - just enough to make it less likely to run away from the pad. A residual pressure valve will have this effect too. Alternatively, if the pistons are extending a fair way with new, full thickness pads to contact the rotor, try using a shim between pad and piston to take up the clearance and see if it still happens...

Secondly, if you're having trouble drawing fluid from the reservoir, it's likely that your system would be tough to bleed properly. Rebleed it after fixing the master cyl problem, being sure to take the calipers off their mounts and rotate them to bleed in different positions to allow any air trapped anywhere in the caliper to find its way out to the bleed point.

Or, it could be something else entirely...

[Alejandro]

I dont think that the weight of the fluid is retracting the pistons..because that negative pressure that appears when you release the pedal is compensated by the fluid in the reservoir that is sucked into the system.

But the main problem here is if you can lock all the wheels..So make sure of that.

Also.. smaller the master cylinder diameter.. longer the pedal travel.

[rwolcott23]

If it is not the weight of the fluid then what could it be? I looked this morning and the pistons retract about 1/32" or more. All of the tolerances and fits were taken directly from a Yamaha caliper, which is where we got the pistons, seals, etc...

Bob

[BryanH]

Before doing anything radical, try removing a brake pad and carefully pump the pistons out 50%, then push them back just far enough to refit the pad. Do this to each pad one at a time. If retraction is still excessive its prob. due to incorrect seal groove dimensions or angle.
Residual line pressure valves prevent entry of air in drum brake systems and normally cause drag in a disc brake system
AP sell springs to limit pull back. the spring locates on a .600" x .103" spigot on the back of the piston

[James Waltman]

Bob,
You guys made your own calipers and are using OEM pistons and seals right?
The seals are often (usually?) designed to suck the piston back into the housing so they don't drag on the rotor. I suspect that in the stock application the piston draws back about the same amount. This won't cause a spongy pedal. It will just take a small amount of pedal travel before the pistons touch the rotor again – then the pedal should be firm. Think about how much force it should take to move the pistons back out to the rotor – about zero. I don't think that this movement could be confused with a soft pedal. So I bet this isn't your problem unless you are using up all of your pedal travel or master cylinder travel to move the pistons to the rotor.

We have made our own calipers for the longest time and we had some problems last year before we got it all figured out. We started by bench bleeding the master cylinders with all of the lines attached and plugged – no calipers. If you can bench bleed the lines successfully then add the calipers to the system and have them clamp on a small piece of plate that is the same thickness as the rotor. This makes it easy to move the caliper around and gives air bubbles a chance to escape. If you can get a firm pedal then reinstall everything in the car and try again. If you can't get a firm pedal with the entire system on the bench you may have more serious problems.

Look for other areas that can lead to wasted motion. Denny mentioned the pedal mount but it can also be the master cylinder mount or the pedal itself.

We have used the Wilwood Residual Pressure Valve (http://www.wilwood.com/Products/006-MasterCylinders/010-RPV/index.asp)
http://www.wilwood.com/Products/006-MasterCylinders/010-RPV/10-lb.jpg

Let us know what you find out.